In an electric motor control apparatus such as an inverter apparatus using a semiconductor device for electric power, at the time of an operation of the apparatus, the semiconductor device for electric power generates heat and a junction temperature of a chip increases and at the time of a stop of the apparatus, the heat generation stops and the junction temperature decreases. Also, at the time of starting and stopping an operation of an electric motor and the time of a sudden change in a load, an output current of the apparatus changes largely, so that the junction temperature of the semiconductor device for electric power also changes largely. As a result of this, thermal expansion and thermal shrinkage of a chip portion of the semiconductor device for electric power are repeated due to repeats of the operations and stops and repeats of the sudden changes in the load or speed.
On the other hand, the semiconductor device for electric power is generally assembled using various materials with different thermal expansion coefficients. Particularly when a temperature of a junction between a heat spreader and a wire bonding portion or the semiconductor device for electric power increases, wire bonding gradually begins to peel off a chip due to thermal expansion stress of a coating agent of the chip or metal fatigue of a junction material begins to occur due to thermal expansion stress by a difference between the chip and the heat spreader in thermal expansion coefficients. Due to repeated operations and stops and repeats of the sudden changes in the load or speed, finally, the wire bonding peels completely and becomes an open state. That is, it results in failure or breakage of the semiconductor device for electric power. A cycle of thermal expansion and thermal shrinkage to the time when this wire bonding peels completely due to thermal expansion stress and results in failure or breakage is called a power cycle.
Therefore, particularly in an apparatus with high repeating frequency of the operations and stops, for example, in the case of using the inverter apparatus in motor driving of an elevator or an AC servo apparatus, or in an apparatus with sudden changes in the load, for example, in the case of using the inverter apparatus in a compressor, a life of the semiconductor device for electric power shortens due to the power cycle, so that some measures need to be taken.
There is a life monitoring apparatus of a semiconductor device for electric power disclosed in Patent Reference 1 (JP-A-8-51768) as means in which an object is to provide an apparatus capable of monitoring a life of a semiconductor device for electric power due to a power cycle and grasping time of maintenance of the semiconductor device for electric power before the life expires and preventing breakage of the semiconductor device for electric power.
The object of Patent Reference 1 is to protect the semiconductor device for electric power used in an inverter apparatus etc. before resulting in the life due to the power cycle. It is constructed so that from a correlation between a power cycle and a difference between junction temperatures of the semiconductor device for electric power, a power cycle corresponding to the difference between junction temperatures of the semiconductor device for electric power in the inverter apparatus is estimated to be the life and the number of operations of the inverter apparatus is counted by a counter. When a count value exceeds a first reference value, an alarm signal is outputted and when the count value exceeds a second reference value, a trip signal is outputted and the inverter apparatus is stopped forcedly.
In Patent Reference 1, there was a problem that a value of the difference between junction temperatures changes normally but the value of the difference between junction temperatures is set to a fixed value by selecting typical one point in operations and stops of the apparatus and desired accuracy of an estimated life cannot be obtained.
Also, there is Patent Reference 2 (JP-A-8-126337) as means in which an object is to obtain an inverter apparatus capable of taking life-prolonging measures, for example, improving a use method before reaching a life of a semiconductor device for electric power.
According to this reference, alarm processing such as an alarm display command can be performed when the number of thermal stresses obtained from the number of thermal stresses of temperature change amplitude computed based on amplitude in changes in an estimated temperature of the semiconductor device for electric power and the number of thermal stresses of temperature change ratio computed based on ratio in changes in an estimated temperature of the semiconductor device for electric power exceeds the number of allowable thermal stresses. Also, one can obtain the residual life time from the number of thermal stresses and the number of allowable thermal stresses to execute a display command. Alarm processing such as an alarm display command performed when operation cannot be performed by expected life time with operation of set time when the number of thermal stresses every set time obtained from the number of thermal stresses of temperature change amplitude every set time and the number of thermal stresses of temperature change ratio every set time exceeds the number of allowable thermal stresses per set time and also, obtaining an operable life with operation of set time to execute a display command are also described in Patent Reference 2.
In Patent Reference 2, a part which has reached fatigue is displayed by life estimation due to thermal stress and a worker can easily decide the part to prevent a fault and also by estimating whether or not operation can be performed by expected life time with operation of set time, the worker can improve a use method or a load state and use frequency of the inverter apparatus and thereby take life-prolonging measures. Unless the worker inspects a display part and checks its display or alarm and checks a life determination result or a life estimation result, the life-prolonging measures cannot be taken. There was also a problem that an abnormal stop of a system is made since the inverter apparatus stops an output by a life determination without taking the life-prolonging measures in case of missing the display or alarm.
This invention is implemented to solve the problems as described above, and a first object is to obtain an electric motor control apparatus capable of doing life estimation with high accuracy.
Also, a second object is to obtain an electric motor control apparatus capable of satisfying a set expected life by automatically decreasing amplitude of changes in temperature of a semiconductor device.